1. Field of the Invention
The present invention relates to a method for operating a blast furnace, and more particularly to a method for operating the blast furnace wherein pulverized coal is blown in through tuyeres of the blast furnace.
2. Description of the Prior Arts
It has been customarily practiced that pulverized coal is blown in together with hot blast air through tuyeres of a blast furnace to be substituted partially for cokes introduced through a furnace top into the blast furnace. This substitution amount for the cokes, however, is 50 to 60kg per molten pig iron ton on the ground that the following is taken care of:
(a) Flame temperature at a nose of the tuyere goes down because endothermic reaction of volatile matters contained in the pulverized coal occurs due to decomposition of the volatile matters in advance of combustion of the pulverized coal; and
(b) the pulverized coal is put into perfect combustion at the vicinity of the nose of the tuyere.
Furthermore, in order to improve blast furnace productivity, recently, various reports of allowing blast gas blown in through the tuyeres to be composed mainly of oxygen have been made. For example, a Japanese Patent Application Laid Open (KOKAI) No. 159104/85 discloses a method wherein:
(1) Through a furnace top, burdens composed mainly of iron ores and cokes are charged into a blast furnace;
(2) Through tuyeres, pure oxygen, pulverized coke and temperature control gas which restrains flame temperature at the tuyere nose from rising are blown in;
(3) Through an intermediate level of the blast furnace, preheating gas which is free substantially from nitrogen is blown in to preheat the burdens; and
(4) By means of the pure oxygen blown in, the cokes included in the burdens are burned to melt and reduce the iron ores charged as well as to generate a blast furnace gas which substantially free from nitrogen from the furnace top.
This method, however, has been very hard to last a stable operation of the blast furnace through a long period.